If you can increase your RAM to 2 GB. That is the sweet spot for XP. Never want to skimp on RAM. This will probably at least double the speed of your computer because right now, most of your memory is actually being stored on the hard drive. The processor isn't bad for XP even though its a single core. RAM would be the largest sticking point. No one should be forced to use a computer with less than 2 GB of RAM. Upgrading to 2 GB will cost about $75.00 plus shipping at
www.newegg.com. You can't go over 3 GB though on XP, even though the board supports 4 GB.
Look at Newegg.com for products and call them directly if you have questions if something works for your particular model. One thing to note about Dell is that they do not plan their machines for expansion. The power supply provided will give you enough power for what you buy but not additional cards or devices. If you add a video card, hard drive or new optical drive, then you will most likely need to improve the power supply as well.